Strange grinding - goes away if print is not oriented normal to x-y axis.

[gdog 4]

I am asking for an acquaintance who has a UM3 Extended.  The issue is that when printing lines that are parallel to the x or y axis, the printer is making a grinding noise.  I am told that re-orienting the print in Cura so that the walls are at an angle (i.e., both x and y motors would be running to print a 45-degree line across the print bed) the grinding goes away.  This makes no sense to me. 

 

I hope to get to check it out personally, but figured someone here might know what could cause such a condition.

 

Thanks in advance for any help!

 

UPDATE:  I went to see the machine in action.  Indeed with even a small change in the alignment of the model the machine prints as smooth and quiet as the day it came out of the box.  Examining and thinking about it a bit more, my current idea is that as the slicer sets (and the hardware attempts to maintain)  a print speed, when nozzle is moving strictly parallel to either x or y axis, the respective motor must move the gantry at the desired speed on it's own.  When, however, the head is moving in both x and y axis, each motor can contribute a lower component velocity according to angle and the "hypotenuse" vector of print head speed.  Therefor the motor has less friction to overcome and would possibly not be as noisy.  The owner of the machine said she contacted Ultimakers support people and they suggest that she disassemble the machine and lubricate the motors.  So that would fit with this assessment that the motors are not able to function at their peak speeds anymore.   The test will be to make a print job that forces slower print speeds but maintains the orientation that is normal to the x-y axis. 

Edited March 25, 2022 by gdog
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